Various implantable medical devices are advantageously inserted within various body vessels, for example from an implantation catheter. Minimally invasive techniques and instruments for placement of intralumenal medical devices have been developed to treat and repair such undesirable conditions within body vessels, including treatment of venous valve insufficiency. Intralumenal medical devices can be deployed in a vessel at a point of treatment, the delivery device withdrawn from the vessel, and the medical device retained within the vessel to provide sustained improvement in vascular valve function. For example, implantable medical devices can function as a replacement venous valve, or restore native venous valve function by bringing incompetent valve leaflets into closer proximity. Such devices can comprise an expandable frame configured for implantation in the lumen of a body vessel, such as a vein. Venous valve devices can further comprise features that provide a valve function, such as opposable leaflets.
Implantable medical devices can comprise frames that are highly compliant, and therefore able to conform to both the shape of the lumen of a body vessel as well as respond to changes in the body vessel shape. Dynamic fluctuations in the shape of the lumen of a body vessel pose challenges to the design of implantable devices that conform to the interior shape of the body vessel. The shape of a lumen of a vein can undergo dramatic dynamic change as a result of varying blood flow velocities and volumes there through, presenting challenges for designing implantable intralumenal prosthetic devices that are compliant to the changing shape of the vein lumen.
For some applications, an implantable frame having a radial strength that varies over time upon implantation is desirable. In particular, optimizing the degree to which a medical device for implantation within a body vessel is compliant to changes in the shape of the body vessel can involve consideration of various factors. For example, a medical device comprising a highly compliant frame can minimize distortion of a body vessel by being highly responsive to changes in the shape of the body vessel.
For treatment of many conditions, it is desirable that implantable medical devices comprise remodelable material. Implanted remodelable material provides a matrix or support for the growth of new tissue thereon, and remodelable material is resorbed into the body in which the device is implanted. Common events during this remodeling process include: widespread neovascularization, proliferation of granulation mesenchymal cells, biodegradation/resorption of implanted remodelable material, and absence of immune rejection. By this process, autologous cells from the body can replace the remodelable portions of the medical device.
Mechanical loading of remodelable material during the remodeling process has been shown to advantageously influence the remodeling process. For example, the remodeling process of one type of remodelable material, extracellular matrix (ECM), is more effective when the material is subject to certain types and ranges of mechanical loading during the remodeling process. See, e.g., M. Chiquet, “Regulation of extracellular matrix gene expression by pressure,” Matrix Biol. 18(5), 417-426 (October 1999). Mechanical forces on a remodelable material during the remodeling process can affect processes such as signal transduction, gene expression and contact guidance of cells. See, e.g., V C Mudera et al., “Molecular responses of human dermal fibroblasts to dual cues: contact guidance and mechanical load,” Cell Motil. Cytoskeleton, 45(1):1-9 (June 2000). An earlier study by C A Tozzi et al, found that: (1) pulmonary vascular endothelial cells responded to mechanical tension by producing PDGF-like material and (2) a 4-hour application of 50 mmHg hydrostatic pressure to cultured pulmonary artery endothelial cells induced v-sis expression, suggesting that “certain vascular cells can respond to an applied load by elaborating factors that affect growth and matrix production of surrounding cells in the blood vessel wall.” See C A Tozzi et al., “Pressure-induced connective tissue synthesis in pulmonary artery segments is dependent on intact endothelium,” J Clin Invest. 84(3), pp. 1005-1012, 1011 (1989).
Therefore, a highly compliant frame with minimal radial strength may provide inadequate mechanical loading to material attached to the frame to allow or promote certain desirable processes to occur within the attached material, such as remodeling, or within the body vessel. In some instances, frame radial strength can be a trade-off between enabling the remodeling of material attached to the frame, and minimizing the distortion or disruption of the body vessel. Implantable endolumenal stent frames comprising a tubular, radially compressible and axially flexible structure having one or more controlled fracture initiation sites have been disclosed, for example, in published U.S. patent application Ser. No. 10/742,943 by Stinson, published as US2004/0138738 A1. However, there still exists a need in the art for an implantable prosthetic device frame that is capable of balancing concerns of conforming to the shape of a body vessel lumen and providing optimal tension on a remodelable material attached to the frame.
What is needed are medical devices that provide a radial strength that changes over time so as to provide a reduced amount of tension on a remodelable material after implantation within a body vessel for a desired period of time.